1. Field of the Invention
This invention relates to respiratory therapy devices, and more particularly, to the design and construction of an exhalation valve assembly which can be modified for use in various volume ventilator circuits.
2. Prior Art
Volume ventilator circuits utilize an exhalation valve assembly to hold and maintain pressure within the circuit and selectively allowing gases to be exhaled by the patient and to escape therefrom. Such valve assemblies are comprised of a valve body and have a gas inlet conduit, which forms a gas discharge port within the valve body, and a gas outlet conduit. A flexible diaphragm selectively closes off the discharge port during inspiration. When the patient exhales, the diaphragm is pushed away from the port so as to allow the exhaled gases to escape from the valve body through the gas outlet conduit.
The pressure holding capability of a volume ventilator circuit is dependent upon a number of factors including the ratio of the area of the diaphragm which extends across the chamber of the valve (hereinafter referred to as the "effective area") to the area of the gas discharge port. For ease of reference, the ratio of the effective area of the diaphragm to the area of the discharge port is referred to herein as the "valve area ratio."
One of the most widely used volume ventilators in the respiratory therapy field has a limited capability for holding elevated Positive End Expiration Pressure ("P.E.E.P.") when using a circuit with a valve assembly having a valve area ratio usually below 1.5. The valve assembly used in such a circuit is specifically designed to achieve this ratio and cannot be modified so as to be used in another circuit requiring a different ratio.
Another popular volume ventilator works on a somewhat different principal. This machine is not dependent upon the valve area ratio for high P.E.E.P. pressures, but is dependent on the valve area ratio for low patient exhalation effort beyond P.E.E.P. pressures. To achieve the required valve area ratio, the valve assembly is specifically designed and cannot be modified to provide a different valve area ratio for use in a different machine.
Thus, prior art volume ventilators have required the use of specifically designed valve asemblies in order to achieve the desired valve area ratio. Notwithstanding the increased costs of manufacturing one specific valve for one type of machine and yet another valve for another type of machine, the prior art has been unable to provide any interchangeability of such valve assemblies.
In other pressure circuits, the valves are also specifically designed for each circuit. An example of a prior art valve assembly for use in a pneumatic control system is shown in U.S. Pat. No. 3,633,605. This valve assembly employs a flexible diaphragm which selectively closes off one of two inlet ports. When the pressure from gas entering one inlet port is greater than that in the other inlet port, the diaphragm is pushed away from that port with the higher pressure, opening it to gas flow. The other port is occluded, thereby preventing gas flow. This valve assembly is designed such that there is no easy way to change the valve area ratio without constructing a whole new housing and diaphragm each having different dimensions. Because of this fact, such valve assembly cannot easily be used in a different system requiring a different valve area ratio.
Yet another valve assembly is disclosed in U.S. Pat. No. 3,419,031. The valve shown in that patent also suffers from the above identified limitations. More specifically, such valve assembly includes a specifically designed resilient valve element which has dimensions related to the dimensions of the inlet and outlet conduits. As discussed hereinabove, it is not readily apparent how one could alter such assembly so as to use the valve in a different system.
Thus, the prior art valve assemblies suffer from the shortcoming of using a specifically configured valve assembly. This leads to a proliferation of valve assemblies all fundamentally designed to perform the same function. The present invention overcomes these problems by providing a valve assembly which can be easily modified so as to achieve different valve area ratios. In this manner, one valve body can be made for use in different environments. The expense of constructing entirely different valve assemblies for use in different pressure circuits is thereby obviated.